1. Field of the Invention
The present invention relates to a ceramic electronic component.
2. Description of the Related Art
These days, mounting of many electronic components, such as ceramic electronic components, on a wiring board integrated within an electronic device is widely performed. Hitherto, lead (Pb) solder has been generally used for mounting electronic components on a wiring board. Lately, however, because of the effect lead has on the environment, mounting of electronic components by using Pb-free solder has been actively attempted. However, since Pb-free solder has a relatively high melting point, high-temperature treatment is necessary for soldering using Pb-free solder. As a result, cracks are likely to occur in a ceramic base of a ceramic electronic component due to the difference in the thermal expansion coefficient between the ceramic base and an outer terminal electrode of the ceramic electronic component. Due to the recent increasing electronification of vehicles and also due to an increase in the in-vehicle living space, the size of the engine compartment in a vehicle is decreasing, and at the same time, an electronic control unit (ECU) is installed closer to the engine or the transmission of a vehicle. Accordingly, electronic components are being used in higher temperature environments. In higher temperature environments, a temperature change varies the amounts of expansion and contraction of terminal electrodes or solder. This may generate a mechanical stress, and thus, cracks may occur in solder itself.
In view of this background, the replacement of solder by conductive adhesive is now being considered. In the conductive adhesive used for this type of purpose, a filler metal made of, for example, Ag, is added to a thermosetting resin, such as an epoxy resin. The thermosetting temperature of the conductive adhesive is lower than the melting point of Pb-free solder. Thus, if a conductive adhesive is used for mounting a ceramic electronic component, it is possible to reduce the thermal stress applied to the ceramic base of the ceramic electronic component. Since the conductive adhesive contains resin having high elasticity, cracks do not occur in the bonding medium itself. An example of a ceramic electronic component that can be mounted by using such a conductive adhesive is disclosed in Japanese Unexamined Patent Application Publication No. 2013-197186.
This publication discloses the following ceramic capacitor as a ceramic electronic component that can be mounted by using a conductive adhesive. An outer electrode of the ceramic capacitor includes a first metal layer and a second metal layer. The first metal layer is formed as a first layer of the outer electrode by applying a Cu-containing conductive paste to a ceramic base of the ceramic capacitor and baking it. The second metal layer is formed as a second layer (outermost layer) of the outer electrode by applying an Ag—Pd-containing conductive paste to the first metal layer and baking it. Through this electrode structure, the mounting of the ceramic capacitor using a conductive adhesive is implemented.
As in the electrode structure disclosed in this publication, in a ceramic electronic component including first and second metal layers formed in a manner described above, when forming the second metal layer, thermal diffusion may develop between the second metal layer and the first metal layer, which may cause the formation of porosities in the second metal layer due to the Kirkendall effect. Due to these porosities, the sealing characteristics of the second metal layer for sealing the first metal layer may be impaired, which may decrease the moisture-resistance reliability. Accordingly, because of this need to suppress such thermal diffusion, in the electrode structure disclosed in the above-described publication, it is not possible to sufficiently raise the baking temperature for forming the first and second metal layers. Thus, the second metal layer (outermost layer) is not sufficiently sintered, thereby decreasing the density of the electrode. In the second metal layer having a low density, very small holes may be formed, and these small holes may reach the first metal layer, depending on the location.
If the above-described type of electronic component is mounted on, for example, the ECU in a vehicle, by using conductive adhesive, exhaust gas from the vehicle or gas generated from a lubricant in the engine or the transmission may enter and be sucked into the small holes formed in the second metal layer (outermost layer) and the first metal layer (underlying layer). The gas sucked in this manner contains oil components, and sulfur compounds contained in the oil components may sulfurize the second metal layer and corrode it. During the corrosion, the metal in the second metal layer may melt into the oil components and diffuse in them, and may precipitate and reach the surface of the second metal layer. If this phenomenon continues for a long time, the entire metal in the first metal layer may precipitate and reach the surface of the second metal layer (outermost layer), which may result in a situation where there is no metal left in the first metal layer.